This invention relates to superalloys exhibiting superior mechanical properties, and more particularly to superalloys useful for high temperature, high strain applications, such as components of aircraft gas turbine engines.
Nickel-base superalloys are well known for their superior mechanical strength at high temperatures. As a result, such alloys have been beneficially employed in aircraft gas turbine engines to permit higher temperature operation and improved efficiency.
However, there is a recognized need in both the aerospace and power generation gas turbine industry for lower cost advanced technology materials. More specially, there is a need for the development of advanced superalloy materials and manufacturing processes that make it possible to produce affordable, integrally bladed turbine wheels exhibiting significantly increased low cycle fatigue (LCF) life and improved airfoil stress rupture life.
Traditionally, the discs or hubs of gas turbines have been formed in a forging process, and the blades in a casting process. The blades are then attached to the disc or hub mechanically. The reason for using separate forming processes is that the discs or hubs preferably have an equiaxed grain structure, giving them maximum tensile strength and low cycle fatigue properties. Preferably, the blades should have a directionally solidified (DS) columnar grain structure, or even a single crystal structure, in order to avoid high temperature creep failure created by lateral grain structure, i.e., grain structure extending transverse with respect to the longitudinal axis (major stress direction) of the blade.
Techniques have been developed to integrally cast the blade and hub in such a way as to obtain directionally solidified, columnar grain blades and equiaxed grain hubs for small integral turbine wheels. Unfortunately, the alloys currently available are better suited to form either an equiaxed grain structure or a directionally solidified, columnar grain structure. High creep strength alloys have not been available which perform well in both grain structures.
As a result, the integrally cast blade and hub gas turbine wheels which have heretofore been utilized commercially have utilized an equiaxed grain structure.
The present invention provides nickel-base superalloys that perform well in both an equiaxed and directionally solidified, columnar grain structure. These alloys exhibit increased grain boundary strength and ductility while maintaining microstructural stability. The improved grain boundary strength and ductility allow both directionally solidified columnar grain casting and equiaxed casting of an integrally bladed cast turbine wheel that will provide superior capabilities at a substantially lower cost when compared to conventional turbine wheels having blades that are separately cast and mechanically attached to a forged turbine disc.
The nickel-base alloys associated with this invention are particularly characterized by a relatively low titanium content and a relatively high tantalum content. The relatively low titanium content (about 0.25% by weight or less) reduces decomposition of titanium carbides during the necessary post-cast hot isostatic pressing (HIP). The relatively high tantalum content of 5.9-6.3 by weight produces grain boundaries comprising of discrete tantalum carbides that remain stable upon hot isostatic pressure treatment, and therefore preserves high grain boundary strength and ductility after the hot isostatic pressure treatment. Although a low titanium content is desired, it has been found that some titanium is needed (at least about 0.05% by weight) to provide excellent fatigue crack growth resistance. Similarly, the tantalum content should not be either too high to too low. The nickel-base alloys of this invention are also characterized by a relatively high refractory element content (tungsten, tantalum, rhenium and molybdenum).